The present invention relates to curable silicone rubber compositions. More particularly, the present invention relates to one-component, alkoxy-functional, room temperature vulcanizable (RTV) silicone compositions which exhibit low modulus upon curing. Processes for making such low modulus silicone compositions are also provided.
Elastomeric siloxane compositions capable of vulcanizing at room temperature have achieved considerable commercial success. In certain applications, such as high rise building construction, it is desirable and often critical to utilize low modulus sealants for adhering the window panes to the metal frame of the building. This low modulus property enables the silicone elastomer to easily compress and expand with building movement due to winds and the like without causing cohesive or adhesive failure. In addition to being a low modulus composition, it is often important that the silicon composition also have high tensile strength and high elongation. These properties will ensure that the silicone elastomer can accommodate, for example, plus or minus 50% compression or extension in the joints without failing. Of course, the more the sealant can expand and compress in terms of thickness of the joint, the more desirable it is for use as a construction sealant.
Because of the recent emphasis by architects on "mirrored" high rise buildings, that is, the exterior of the building appears to be a large mirror, for both aesthetic and energy-saving reasons, there has been a great deal of interest in providing suitable low modulus silicone sealants. Perhaps the most common type of low modulus silicone sealant is based on a silanol endblocked polydiorganosiloxane in combination with any of a number of crosslinking and/or coupling agents which impart low modulus to the cured composition.
Murphy, U.S. Pat. No. 3,341,486, describes room temperature vulcanizable organopolysiloxane compositions comprising a silanol terminated polydiorganosiloxane and a mixture of a difunctional silicon-nitrogen material having attached to silicon two radicals of the unit formula ##STR1## and a polyfunctional silicon-nitrogen material attached to silicon having at least three aminoxy radicals of the unit formula EQU .tbd.SiOY
where Y is a monovalent amine radical and R" is a monovalent hydrocarbon radical, alkoxy radical or triorganosiloxy radical.
Golitz, U.S. Pat. No. 3,417,047, relates to organopolysiloxane compositions curable at room temperature to elastomers comprising a silanol endstopped polydiorganosiloxane and an organosilicon crosslinking agent having the formula ##EQU1## where X is hydrogen, alky or aryl, m equals 1 to 4, b has a positive value of at least 3 per molecule of organosilicon compound and Y is a Si--N bonded carboxylic acid amide radical.
Murphy, U.S. Pat. No. 3,441,583, discloses aminoxyorganosilicon materials useful as curing agents and coupling agents in silanol-containing organopolysiloxane compositions.
Klebe, U.S. Pat. No. 3,488,371, involves the production of linear difunctional silylamides which are useful in the preparation of linear polysiloxanes and which cure to rubbery materials upon exposure to moisture at room temperature.
Clark et al., U.S. Pat. No. 3,766,127, discloses a composition stble under moisture free conditions and curable to a low modulus silicone elastomer comprising a mixture of 100 parts by weight of a hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a non-acidic, non-reinforcing filler; 3.5 to 8 parts by weight of ##STR2## in which R is methyl or vinyl, R' is methyl, ethyl or phenyl and 0.3 to 4.2 parts by weight of ##STR3## in which R' is defined above and R" is the same as R.
Brady et al., U.S. Pat. No. 3,766,128, relates to a low modulus room temperature vulcanizable silicone elastomer obtained by curing a mixture of 100 parts by weight of a hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a non-acidic, non-reinforcing filler; 2 to 7.5 parts by weight methylvinyldi-(N-methylacetamido)silane and 0.5 to 4 parts by weight of an aminosilane of the formula RSi(NR'R").sub.3 in which R is alkyl, phenyl or vinyl, R' is hydrogen or alkyl and R" is alkyl or phenyl.
Toporcer et al., U.S. Pat. No. 3,817,909, describes low modulus room temperature vulcanizable silicone elastomers obtained by mixing 100 parts by weight of a hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a non-acidic, non-reinforcing filler; 2 to 20 parts by weight of ##STR4## in which R is methyl, vinyl or phenyl and R' is methyl, ethyl or phenyl and 0.25 to 7 parts by weight of an aminooxysilicon compound having 3 to 10 aminoxy groups per molecule.
Klosowski, U.S. Pat. No. 3,996,184, relates to a one package, low modulus, room temperature vulcanizable composition obtained by mixing 100 parts by weight of a hydroxyl endblocked polydimethylsiloxane; 0 to 200 parts by weight of a nonacidic, non-reinforcing filler; 2.5 to 10 parts by weight of ##STR5## in which R is methyl or phenyl; 1 to 6 parts by weight of an aminoxysilicon compound having 3 to 10 aminoxy groups per molecule; and 1 to 5 parts by weight of N,N-dimethylformamide, acetonitrile, or N-n-butylacetamide.
Beers, U.S. Pat. No. 4,100,129, discloses self-bonding, low modulus, one-package room temperature vulcanizable silicone compositions comprising a silanol chainstopped polydiorganosiloxane; a crosslinking silane; and a silanol reactive organo-metallic ester compound of a metal, the compound having radicals attached to the metal atom, at least one of the radicals being a substituted or unsubstituted hydrocarbonoxy radical, attachment being through an M--O--C linkage where M is the metal, and any remaining valences of M are satisfied by other organo radicals attached to M through such linkages, or an --OH, --O--, or M--O--M linkage, the weight ratio of the organometallic ester to silane being at least 0.5 to 1.
Beers, U.S. Pat. No. 4,323,489, teaches a room temperature vulcanizable silicone rubber composition with low modulus comprising a silanol endstopped diorgaopolysiloxane, a difunctional acetamide coupler, and as the crosslinking agent, a minor amount of compound selected from ketoxime functional and aminoxy functional silanes and mixtures thereof.
Hahn, U.S. Pat. No. 4,360,631, describes a flowable, low modulus, room temperature vulcanizable silicone composition obtained by mixing 100 parts by weight of hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a nonacidic, non-reinforcing filler; 2 to 20 parts by weight of methylvinyldi-(epsilon-caprolactamo)silane and 0.25 to 7 parts by weight of an aminoxysilane compound having 3 to 10 aminoxy groups per molecule.
Lampe, U.S. Pat. No. 4,410,677, provides a low modulus, room temperature vulcanizable silicone rubber composition with a good shelf life comprising a silanol containing polysiloxane, a filler, an acyloxy functional silane as the crosslinking agent, and as the catalyst, a compound selected from the group consisting of zinc salts and zirconium salts, or a co-catalyst system comprising as one co-catalyst a tin salt of a carboxylic acid and as the other co-catalyst, either a zinc salt or a zirconium salt of a carboxylic acid.
Those skilled in the art will readily appreciate that all of the foregoing disclosures depend on the use of silanol(monofunctional) terminated polydiorgaosiloxanes. Such siloxanes are easily chain-extended, e.g. coupled, upon exposure to atmospheric moisture to obtain low modulus silicone rubber compositions.
Another class of silicone rubber compositions is based on polyalkoxy(polyfunctional) terminated polydiorganosiloxanes as originally disclosed by Brown et al. in U.S. Pat. No. 3,161,614. Such compositions, however, are not easily made to be low modulus upon curing because they already contain terminal groups which will effect crosslinking rather than coupling. Accordingly, it is desirable to provide low modulus polyalkoxy-functional silicone compositions.
Cooper et al., U.S. Pat. No. 3,542,901, teaches that one-component silicone RTV compositions of the type disclosed by Brown et al. are ineffective for certain sealing applications because the modulus is too high. Cooper et al. goes on to disclose a method to provide compositions vulcanizable to low modulus rubbers comprising utilizing a polydiorganosiloxane having therein a proportion, up to 40 percent, of terminal units of the formula --SiR.sub.3, the remaining terminal units being alkoxy functional. Preferably the polydiorganosiloxane has a viscosity within the range of 2,000 to 20,000 centipoise at 25.degree. C.
Beers, U.S. patent application Ser. No. 349,537, filed Feb. 17, 1982, discloses that alkoxy functional RTV compositions of the type disclosed by White et al., U.S. Pat. No. 4,395,526, and Dziark, U.S. Pat. No. 4,417,042, can be made low modulus by adding thereto from 2 to 20 parts by weight of a plasticizer fluid containing a high degree of trifunctional siloxy units or a mixture of trifunctional and tetrafunctional siloxy units. Optionally, additional plasticizers such as linear triorganosiloxy endstopped polydiorganosiloxanes having a viscosity ranging from 10 centipoise to 20,000 centipoise can be included in the composition.
All of the foregoing patents and patent applications are incorporated by reference into the present patent application.
Although a wide choice is thus seen to be available for the preparation of low modulus, one-component RTV compositions, there has not been much appreciation by those skilled in the art of the factors involved in securing specific properties in the cured silicone rubber, especially when the polysiloxane is polyalkoxy functional. It has now been discovered that low modulus, silicone rubber compositions can be prepared from polyalkoxy functional polydiorganosiloxanes by employing therewith certain silanes having attached thereto two nitrogen-containing radicals.